Study On Optimization And Evaluation Of Heating Tower Heat Pump System

A heating tower heat pump(HTHP),as a novel integrated heating and cooling unit,is drawing more and more attention due to its high efficiency,low topographical limitation,and frost-free characteristic.However,lacking heat and mass transfer coefficients and design method of heating tower,system operation characteristics,optimization and performance evaluation of HTHPs limits their applications.To address the above issues,this thesis carries out investigations on heat and mass transfer characteristics,design method,system operation characteristics,optimization and performance evaluation of HTHPs.Theoretical,experimental,and simulation method are adopted in this research,which can make a contribution in the design,operation,and evaluation of HTHPs.Firstly,this thesis experimentally studies the heat and mass transfer characteristics of the heating tower under heat absorption,heat rejection and regeneration conditions.The effects of the air and solution/water inlet parameters on the performance indices are investigated.Based on the experimental results,the correlation expressions of heat and mass transfer coefficients are developed.Then,a heating tower design method adopting tower demand curve and characteristic curve is proposed.In addition,performance comparison between winter and summer conditions are conducted.The heat transfer capacity in summer condition is 2.7 times of that in winter condition,which is caused by the difference of heat and mass transfer driving forces.The self-regeneration process of solution in the operation of heating tower is also presented,which can significantly reduce the solution regeneration demand.Secondly,a HTHP system is established and its performance in both winter and summer conditions is studied.The solution self-regeneration process often occurs in the afternoon when outdoor temperature is high.When the outdoor temperature is low,the solution can absorb more letant heat to satisfy the total heating capacity.These two processes are benefit from the energy storage characteristics of solution.In summer condition,the mean approach and range are 1.3°C and 3.7°C,respectively.This indicate that the heat exchange area of heating tower is surplus in summer condition.Based on the experimants,physics models of the HTHP system are developed and verified,which can provide a basis for the optimization and evaluation of the HTHP system.Thirdly,a model-based optimization method is proposed,which realizes the operation optimization of the HTHP system by combining genetic algorithm and penalty function.Artificial neural network(ANN)models are developed and trained with vast amount of operational data generated by the physics-based model.The ANN models are found to be highly accurate(within 1% relative errors)and computationally efficient(about 300 times faster than the physics-based model).After that,an approach which optimizes both the load distribution and the speed of fans and pumps is proposed.The optimization is implemented by using the ANN models,proposed approach,genetic algorithm and penalty function via a case study.The results show that the energy saving is 14.8% in the cooling season and 4.7% in the heating season.Moreover,the thermodynamic performance in typical days is analyzed to investigate how energy savings could be achieved.Lastly,a large-scale comprehensive performance evaluation of the HTHPs is carried out through the process of location selection,building load calculation,system sizing,simulation,and evaluation.According to the ASHRAE standard,869 typical locations are selected.These locations are in the Zone 3(warm),Zone 4(mixed),and Zone 5(cool),where buildings need both cooling and heating supply.Unified design standards are proposed,and the system sizing is conducted based on the standards.Then the hourly simulations of the HTHP systems in the selected 869 locations are carried out,and seven performance indices(e.g.annual ,regeneration ratio,matching degree)are figured out.The results show that the mean annual of Zones 3–5 in are 4.67,3.68,and 3.11,which indicates that the HTHPs have good performance in these regions.Both the above evaluation method and results can make a contribution in the application of HTHPs in these regions.